The invention relates to electronic device fabrication, and, more particularly, to semiconductor cleaning.
Fabrication of integrated circuits on a wafer of semiconductor material such as silicon typically involves sequences of processing steps such as masked introduction of dopants (impurities effecting electrical characteristics) by ion implantation or diffusion, formation of insulating layers by oxidation or deposition followed by masked etching to make patterns, and formation of conductors by deposition of polycrystalline silicon (polysilicon) or metals also followed by masked etching to make patterns. And prior to each of these steps affecting the wafer surface, the surface must be cleaned to get rid of process chemicals from the prior steps in order to insure that the prescribed reaction occurs or the prescribed material interface arises. Contaminants and irregularities can affect electrical behavior, and cleanups may themselves introduce contaminants including heavy metals, alkali metals and light elements. Cleanups may also be the source of submicron sized particulates that are difficult to detect.
Wafer cleanups should remove gross organics and particulates, remove organic films, remove surface-adsorbed ions and plated metal contaminants, and sometimes remove surface oxide. Standard wafer cleanups include mechanical scrubbing in a liquid and ultrasonic agitation in deionized water (particulate removal), chemical cleaners such as solutions of "piranha" and "RCA cleanup" and choline cleanup, and dry cleaners such as ozone with ultraviolet light. Piranha is a solution of hydrogen peroxide (H.sub.2 O.sub.2) and sulffric acid (H.sub.2 SO.sub.4), and choline cleanup uses a solution of H.sub.2 O.sub.2 with choline ((CH.sub.3).sub.3 N(CH.sub.2 CH.sub.2 OH)OH) at 50.degree. C. followed by an ultrasonic clean in deionized water and a deionized water rinse plus spin dry. RCA cleanup has up to three steps: first a removal of gross organics with perchloroethylene, next a removal of residual organic films with a basic solution of H.sub.2 O.sub.2 and NH.sub.4 OH followed by deionized water rinse and spin dry, and lastly a removal of metallics with an acidic solution of H.sub.2 O.sub.2 and HCl again followed a deionized water rinse and spin dry. The solutions are used at 75-80.degree. C. and essentially provide an oxidizing and complexing treatment which does not attack silicon or silicon dioxide (oxide). The basic solution cleanup alone is frequently called SC-1 and the acidic solution cleanup alone is called SC-2.
Current cleanup systems typically rely on the RCA cleanup as a pre-furnace cleanup. However, once a wafer has been metallized, then both the SC-1 and SC-2 cleanups must be abandoned in favor of other cleaners in order to avoid etching desired metal. The recent use of titanium nitride (TiN) cladding for aluminum or for coated contacts, gates, and emitters or as a standalone "local interconnect" conductor has created a problem in that both the SC-1 basic cleanup and the SC-2 acidic cleanup attack TiN. Thus cleanup for metallized wafers relies on solvents, such as N-methylpyrrolidine (NMP), or ultrasonic or spin rinse cleaning with deionized water. (Note that many organic solvents are flammable and thus cannot be used in ultrasonic cleaning baths due to the potential for explosion). But these cleanups have limited efficacy, and there is a need for better cleanups for metallized wafers, especially with exposed TiN.